Vertical velocity control for aircraft



July 3, 1 R. HOFSTADTER VERTICAL VELOCITY CONTROL FOR AIRCRAFT Filed July 22, 1946 Patented July 3, 1951 VERTICAL VELOCITY CONTROL FOR AIRC RAFT

Robert Hofstadter, Forest Hills, N. Y., assignor to The Norden Laboratories Corporation, New York, N. Y., a corporation of Connecticut Application July 22, 1946, Serial No. 685,342

14 Claims.

My invention relates to a vertical velocity control for aircraft, and more particularly to a control system for controlling an airplane to provide for glide or climb at a predetermined regulated vertical velocity.

In the automatic pilots of the prior art, an airplane may be controlled automatically to fly a predetermined straight course at a level attitude. It is frequently desirable to control an airplane automatically so that it will climb or glide in order that its vertical velocity will be proportional to a desired function of time or any other desired variable or variables. In most systems of the prior art, the pilot must fly the plane to the desired altitude and then make the automatic pilot operative. Similarly, when coming to a landing or reducing altitude, the automatic pilot must be rendered inoperative and the plane flown to the desired altitude by a human pilot.

One object of my invention is to provide a new and improved control system for aircraft whereby an airplane may be automatically controlled to give it a desired vertical velocity either in ascent or descent.

A further object of my invention is to provide an improved automatic control system for airplanes to produce a desired climb or glide in which overrun is obviated.

Other and further objects of my invention will appear from the following description.

The accompanying drawing, which forms part of the instant specification and which is to be read in conjunction therewith, is a circuit diagram showing one embodiment of my invention.

In general my invention contemplates the balancing of a voltage representing the altitude of the airplane through a differentiating circuit against a voltage proportional to the desired vertical velocity so as to produce a signal which is a function of the desired vertical velocity and in the proper direction, which signal is employed as a control signal for the elevator control surfaces of the airplane.

More particularly, referring now to thedraw: ing, an altimeter I is of the type capable of producing a voltage across resistor 2 which is proportional to the altitude of the plane, such as the altimeter described in Civil Aeronautics Bulletin No. 29, at pages 86 to 90 inclusive. The altimeter construction forms no part of the instant invention, and any suitable altimeter which will produce a voltage which is proportional to the altitude may be employed. The resistor 2 is disposed so that its positive side is connected by conductor 3 to a capacitor 4. The other side of the capacitor is connected by conductor 5 to the grid 6 of the thermionic tube 1, which is shown as a twin triode. A plate battery 8 has its negative terminal connected to ground 9 by conductor 10. The positive terminal is connected by conductors ll, i2, and I3 to anodes I4 and i5 through resistors l6 and H. The cathodes l8 and 19 of the tube 1 are connected by resistor 20 to ground 21. The grid 6 is also connected by conductor 22 to a grid resistor 23, the other side of which is connected by conductor 24 to the midpoint between resistors 25 and 26. Grid 21 of tube 1 is connected by conductor 28 to a grid resistor 29, the other side of which is connected by conductor 30 to a variable arm 3l adapted to contact the voltage dividing resistors 32 and 33, whose values correspond to resistors 25 and 26 respectively. The lower end of resistor 26 and the lower end of resistor 33 are connected to ground 34. The negative side of the altimeter resistor 2 is connected by conductor 35 to ground 34. The positive terminal of battery 8 is connected by conductor 36 and resistor ill to a voltage divider 31, the other side of which is connected to ground by conductor 38. The variable arm 39, which is associated with the voltage divider 31, is connected by conductor 40 to one side of a capacitor 4|, the other side of which is connected by conductor 42 to the grid 6 of the thermionic tube 1. The resistor HI governs the amount of the anticipating voltage and is designed to suitthe particular aircraft with which the control system is used. As current flows through the circuit of anode 14, a potential will be developed across the resistor Hi. This potential is impressed through resistor H5 and is led by conductor 43 across the resistor 44 to conductor 38 and thence to ground. The potential developed across resistor 44 is applied by conductor 45 to the grid 46 of a second twin triode 41. Similarly, when current flows through the anode circuit of anode I5, a voltage will be developed across the resistor IT. This voltage is applied through resistor H4 by conductor 48 to one side of a resistor 49, the other side of which is connected to conductor 38 and ground. The voltage developed across the resistor 49 is applied by a conductor 50 to the other grid 5! of the tube 41. The cathodes 52 and 53 of the tube 41 are connected by a self biasing resistor 54 to conductor 38 and thence to ground.

When current flows through the circuit of anode 55 of tube 41, a potential is developed across resistor 56. This potential is led through resistor 51, through conductor 58, across resistors 58 and 60, to conductor 38, and thence to ground. The resistor 51 is shunted by an anticipating capacitor H3 to compensate for servomotor delay. The resistor 51 may be one having non-linear resistance characteristics such that above a critical voltage the resistance falls considerably and hence acts as a stabilizing resistor. The voltage developed across resistors 59 and 60 is applied by conductor H to the grid 62 of thermionic tube 63. Similarly, when current flows through the circuit of anode 64 of tube 41, a potential is developed across the resistor 65. This potential is led through resistor 66 and conductor 61, through resistors 68 and 69, to conductor 38, and thence to ground. The resistor 66 is paralleled by a capacitor I I2 to serve as an anticipating condenser for the servomotor delay. The resistor 66 is similar to the resistor 51. Potential developed across resistors 68 and 69 is applied by conductor to the grid H of the tube 63. The plate potential existing in conductor I3 is led through winding 12 of a relay, through conductor 13, to the anode 14 of the tube 63. Similarly, the plate potential existing in conductor I3 is led through winding of a relay coil, through conductor 16, to the anode 11 of the tube 63. Upon the energization of relay winding 12 a circuit is closed across contacts 18 and 19 so that current will flow from the battery 80, through conductor 8I, through conductor 82, to an attitude servomotor 83. The servomotor 83 is adapted to rotate a shaft 84 to which the variable arm 39 is attached. It is also adapted to operate an arm 85 which is connected by link 86 to an arm 81 of the gyroscope automatic pilot 88. The automatic pilot 88 produces an output signal which is led by conductors 89 or 90 to a control servomotor 8| which is adapted to drive a control drum 92 to operate control wires 83 and 84 of the airplane elevator 85. When the winding of relay 15 is energized a circuit will be closed across contact points 86 so that current will flow from the battery 80, through conductor 8I, through conductor 81, to operate the attitude servomotor 83 in the opposite direction from that which occurred when current was passed through conductor 82, the circuits being completed in each case to ground 98.

The operation of relay 12 further will close a circuit across contact points 99 and I00. When this occurs, current will flow from the battery 80 across voltage divider IOI, through conductor I02, through conductor I03, through resistor 69, :1

to conductor 38, and thence to ground. It will be observed that resistor 69 is shunted by a capacitor I04 to provide a transient path for a rapid change of potential upon the closing of the circuit. After the capacitor I 04 is charged, the voltage drop across resistor 69 is applied to the grid H to alter its bias. The capacitor I 04 is placed across the resistor 69 and the value of its capacity is such as to give a time constant in the circuit to produce the least sparking. This result is achieved due to the fact that the condenser will hold the voltage at the grid for a fraction of time after the relay opens. In this way the relay acts more positively for longer intervals, both in opening and in closing, thus eliminating chattering and sparking of the relay contact points. The values of the resistance 69 and capacity of the by-pass capacitor I 04 will vary according to the speed of the attitude servomotor the tube 41.

83. Such circuit, however, is needed only for slow servomotors.

Similarly, the energization of relay winding 15 will close the circuit across contact points I05 and I06 whereby current will flow from the battery 80 through voltage divider IOI, through conductor I02, through conductor I01, through resistor to ground. A capacitor I08 similar to capacitor I 04 provides a path for the transient current while the voltage is changing. Upon the closing of the circuit, after the condenser I08 is charged, the voltage drop across resistor 60 is applied to the grid 62 to change its bias.

The common cathode I09 of the tube 63 is connected to ground through variable resistor H0 and conductor 38. The current level which is to flow in windings 12 and 15 of the relays is governed by the value of the resistor IIO.

In operation let us assume that the variable arm 3| is adjusted to the midpoint between resistors 32 and 33 so that resistance 25 equals resistance 32, and resistance 26 equals resistance 33. Let us assume further that the airplane is in level flight at a constant altitude. The voltage drop across resistor 26 is impressed upon grid 6, and the voltage drop across resistor 33 is impressed upon the grid 21. These voltages are equal so that equal small currents flow through the circuits of anodes I4 and I5. Accordingly, there are equal voltage drops across resistors I6 and I1, and equal large potentials applied to both grids 46 and 5I. It follows, therefore, that there will be equal currents flowing through anode circuits of anodes 55 and 64. These anode currents will produce equal voltagedrops across resistors 56 and so that equal small potentials will be applied to grids 62 and II of tube 63. The current levels in the circuits of anodes 14 and 11 of tube 63 are such as to be insuflicient to energize the windings 12 and 15 under these conditions.

The potential developed across resistor 2 of the altimeter I is isolated by the capacitor 4 from the grid 6 so that for a steady state this potential has no effect upon the grid 6.

It will be observed that the circuit will not control the airplane as a function of any predetermined altitude, but the arrangement is such that it will resist changes in altitude due to external factors though it wil not restore the aircraft to any specific altitude.

For example, let us now assume that the airplane fiies through an upcurrent of air so that the altitude of the airplane is increasing. The increase in potential across resistor 2 being a transient is passed through capacitor 4 and applied to the grid 6. This increases the current flowing through the circuit of anode I4 and reduces the positive potential which is applied to grid 46. The increase in the current in anode circuit I4 increases the voltage across cathode resistor 20 so that the current flowing through the circuit of anode I5 is reduced. This reduces the potential drop across resistor I1 and hence increases the potential applied to the grid 5| of Accordingly, the current flowing through the circuit of anode 55 will be decreased, and the current flowing through the circuit of anode 64 will be increased. The decrease of the current of anode 55 will decrease the potential drop across resistor 56 and result in the application of the increased potential upon the grid 62 of control tube 63. The increase in the current flowing through the circuit of anode 64 will in crease the potential drop through resistor 65 and result in the application of a reduced potential upon the grid II of control tube 03. Accordingly, an increased current will flow through the circuit of the anode I4 and through winding 12, energizin the relay to close the circuit across contact points I8 and I9. This permits current to flow throu h conductor 82 and operate the attitude servomotor to rotate shaft 84 in a counter-clockwise direction, moving the link 86 forwardly to control the gyroscope 88 to operate the servomotor 9| to rotate the elevators 95 in a counterclockwise direction, thus controlling the plane to nose it downwardly to counteract the increase in altitude. It will be observed that the above operation places a reduced potential upon the grid II so that when the increase in altitude is corrected, there is apt to be delay which will cause an overrun of the attitude servomotor. Let us now consider the action which results from the closing of the circuit through contact points 99 and I00. When this occurs, positive potential taken from the voltage divider IOI is applied immediately across the capacitor I04, quickly charging it and applying a positive potential upon the grid II. After the capacitor I04 is charged, it discharges through the resistor 69 and the voltage drop across resistor 59 is applied to the rid II, conditioning it to reverse the action of control tube 63 with a minimum of delay. The positive potential furthermore upon grid II increases the current flowing through the circuit of anode 11, which increases the positive bias upon the cathode I09 and thus reduces the current flowing through the circuit of anode 14, again tending to prevent overrun of the attitude servomotor.

Let us now consider the ac ion when the airplane f ies through an ar:a of downdraft which cau es the airplane to lose altitude. The potential across the resistor 2 of the altimeter I will be reduced and the drop in potential is reflected upon. grid 6 since the capacitor 4 will pass transients. The reduction of potential upon the grid 6 of the tube I will reduce the current flowing through the circuit of anode I4, thus reducing the potential drop across the resistor I6 and increasing the potential applied to the grid 46 of tube 50. The reduction in the current through the circuit of anode I4 furthermore reduces the cathode bias which is the potential drop across the resistor 20. This increases the current flowing through the circuit of anode I5 and increases the pot ntial dro across the resistor I! so that a smaller potential is now applied to the grid SI of mixer tube 41. The increase of grid potential upon grid 46 increases the current in t e circrit of anode 55 and increases the potential drop across the resistor 5'6, thus reducing the potential ap lied to the grid I32 of control tube 63. Similarly. the red ction in the current of the circuit of anode 64 reduces the potential drop across resi tor 65. and hence increases the potential applied to the grid II of control tube 63. The increase in potential upon the grid 1| causes an increase in the current flowing through the circuit of anode 11. which includes the winding I5 of the relay. This closes the circuit across contact points 96 and energizes the attitude servomotor 83 to rotate in a clockwise direction, moving the link 86 rearward y and causing the servomotor 9I to rotate the levators 95 in a clockwise direction to bring the nose of the plane upwardly.

The energization of the winding I5 also closes the circuit across contact points I05 and I06 to impress a positive potential across condenser I08 and upon the grid 62 of tube 63 in a manner 6 similarly described above with respect to grid II.

It will be observed that when the airplane moved to a higher altitude by extraneous forces, the shaft 84 was rotated in a, counter-clockwise direction. This moved the arm 39 along the voltage divider 31 so that a more positive potential was impressed upon conductor 40, the transient passing through capacitor 4| to the grid 6. It will be remembered that won the increase in altitude the grid 6 was at an increased potential. The effect of the application of the transient positive potential upon the grid conditions the grid. anticipating its return to a more positive potential by a reduction in the velocity of altitude increas;

In an analogous manner, upon a reduction of the altitude of the airplane, the shaft 84 was rotated in a clockwi e direction, reducing the potential applied to crnductor 40 and hence reducing the potential applied to the grid 6, which is at a decreased positive potential when the altitude of the airplane is dlcreasing. This reduction. in the grid potential of grid 6 anticipates the condition of the grid potential which will be brought about by a reduction of the rate of altitude loss. This arrangement prevents overrun of the aircraft.

Heretofore it was ass=nned that the variable arm 3|, which controls the potential applied t the grid 2! o the tub 1, was at the midpoint between. resistors 32 and 33 in a position balancing the resistors 25 and 26. let us now move the variable arm 3I downwardly to the position shown in the drawing so that the grid 6 is more positive than the grid 21. This condition, it was pointed out above, occurred when the airplane was moved to a higher altitude by external factors such as an updraft. The correction which took place was to place the plane in a glide. When the resistance of resistor 33 is decreased, therefore, the svstem will operate as described above to move the elevators to place the plane in a glide, the rate of descent being proportional to the reduction in potential which is applied to the grid 2 by the variable arm 3I. The control system will operate to place the plane in a glide such that the rate of descent will product a. voltage change across the resistor 2 so that the rate of decrease of the voltage applied to the grid 21 will balance the potential upon grid 6 against the reduced potential applied to grid 21. It will be immediately apparent to those skilled in the art that the variable arm 3| controls the generation of the rate of descent, that is, a rate of decreasing altitude or vertical vslocity in a downward dirzction.

In a similar manner, if the variable arm 3I is moved upwardly to add a portion of the resistor 32 in series wit: the resistor ?3. there will be an increased potential applied to the grid 21. This will produce a condition simulating the descent of the airplane due to external factors, causing the system to operate to place the airplane in a climb. When the rate of climb, that is, the rate of increase of altitude, generates an increas d voltage applied upon grid 6 through capacitor 4 to balance the increased potential placed upon grid 21 by the variable arm 3|, the rate of climb will be constant agreeable to the increased voltage placed upon grid 21.

The system furthermore is such that the rate of climb determined by the incrLased voltage applied to grid 2'! or the rate of glide determined by the decreased voltage applied to grid 27 will "which followed an updraft, described above, will ensue. Similarly, if the rate of glide is greater than that called for by the reduced potential applied to grid 2'! by the variable arm 35, the stabilizing action described above with respect to a downdraft will ensue.

It will be apparent to those skilled in the art that by controlling the movement of brush 3| so as to represent a velocity which is a desired function of any variable, the vertical velocity of the aircraft may be similarly controlled.

It will be seen that I have accomplished the objects of my invention. I have provided an improved control system for aircraft whereby an airplane may be automatically controlled to give it a desired vertical velocity either in ascent or descent. The system is such that the desired climb or glide will be produced in a manner as to obviate overrun. My system furthermore resists changes from the predetermined climb or glide and maintains the airplane at the desired vertical velocity in either direction.

While I have described my invention as a control for an aircraft, it will be obvious to those skilled in the art that it may be used to control the rate of any variable which can be represented as a voltage, as, for example, the motion of a body or the control of a submarine in depth. 7

It will be understood that certain features and sub-combinations are of utility and may be-employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of the claims. It is further obvious that various changes may be made in details within the scope of the claims without departing from the spirit of the invention. It is, therefore, to be understood that this invention is not to be limited to the specific details shown and described.

Having thus described my invention, I claim:

1. A control system for an aircraft including in combination, means for developing a potential proportional to the altitude of an aircraft, a pair of electron discharge devices each having an anode and a grid, cathodes for each of said electron discharge devices, a self biasing common resistor for said cathodes, means for impressing a fixed biasing potential upon the first grid, means for impressing a biasing potential on the second grid, means for impressing a potential which is a functionof the change in said altitude proportional potential with respect to time upon said first grid, aircraft elevator control means responsive to the anode currents of said electron discharge devices, and means for varying the biasing potential on said second grid to determine a desired climb or glide for the aircraft.

2. A control system as in claim 1 in which said elevator control means includes a pair of thermionic tubes having a pair of grids and a pair of cathodes, a self biasing common resistor for said cathodes, a pair of anode resistors disposed in each of the anode circuits of said pair of electron discharge devices, and conductors for impressing the potential across said resistors upon respective grids of said thermionic tubes.

3. A control system as in claim 1 in which said means for impressing a potential which is a function of the change in said altitude proportional potential includes a channel containing a capacitor in series.

4. A control system as in claim 1 in which said elevator control means includes a pair of thermionic tubes having a pair of anodes, a pair of grids, and a pair of cathodes, a common self biasing resistor for said cathodes, resistors in the anode circuits of said first pair of electron discharge devices, respective conducting paths for impressing the voltage drop across respective resistors upon respective grids of said pair of thermionic tubes, a second pair of resistors disposed in respective anode circuits of said pair of thermionic tubes, a pair of control thermionic tubes having-a pair of grids and a common self biasing cathode resistor. and conducting paths for impressing the voltage drops across respectivesecond anode resistors upon respective grids of said control thermionic tubes.

5. A control system as in claim 1 in which said elevator control means includes a pair of thermionic tubes having a pair of anodes, a pair of grids, and a pair of cathodes, a common self biasing resistor for said cathodes, resistors in the anode circuits of said first pair of electron discharge devices, respective conducting paths for impressing the voltage drop across respective resistors upon respective grids of said pair of thermionic tubes, a second pair of resistors disposed in respective anode circuits of said pair of thermionic tubes, a pair of control thermionic tubes having a pair of grids and a common self biasing cathode resistor, and conducting paths including resistors having non-linear resistance characteristics for impressing the voltage drops across respective second anode resistors upon respective grids of said control thermionic tubes.

6. A control system as in claim 1 in which said elevator control means includes a pair of thermionic tubes having a pair of anodes, a pair of grids, and a pair of cathodes, a common self biasing resistor for said cathodes, resistors in the anode circuits of said first pair of electron discharge devices, respective conducting paths for impressing the voltage drop across respective resistors upon respective grids of said pair of thermionic tubes, a second pair of resistors disposed in respective anode circuits of said pair of thermionic tubes, a pair of control thermionic tubes having a pair of grids and a common self biasing cathode resistor, conducting paths for impressing the voltage drops 'across respective second anode resistors upon respective grids of said control thermionic tubes, a pair of relays, means for operating respective relays in response to increases in respective anode currents of said control thermionic tube, and means responsive to the operation of one of said relays for increasing the potential applied to the grid of the control thermionic tube which controls the other of said relays.

7. A control system as in claim 1 in which said elevator control means includes a pair of thermionic tubes having a pair of anodes, a pair of grids, and a pair of cathodes, a common self biasing resistor for said cathodes, resistors in the anode circuits of said first pair of electron discharge devices, respective conducting paths for impressing the voltage drop across respective resistors upon respective grids of said pair of thermionic tubes, a second pair of resistors disposed in respective anode circuits of said pair of thermionic tubes, a pair of control thermionic tubes having a pair of grids and a common self biasing cathode resistor, conducting paths for im pressing the voltage drops across respective second anode resistors upon respective grids of said control thermionic tubes, a pair of relays, means for operating respective relays in response to increases in respective anode currents of said control thermionic tube, means responsive to the operation of one of said relays for increasing the potential applied to the grid of the control thermionic tube which controls the other of said relays, said responsive means including a source of potential, a resistor, means for impressing said potential across said resistor, means for connecting said resistor to said grid, and a capacitor in parallel with said resistor.

8. A control system for an aircraft including in combination, means for developing a potential proportional to the altitude of an aircraft, a pair of electron discharge devices each having an anode and a grid, cathodes for each of said electron discharge devices, a self biasing common resistor for said cathodes, means for impressing a fixed biasing potential upon the first grid, means for impressing a biasing potential on the second grid, means for impressing a potential which is a function of the change in said altitude proportional potential with respect to time upon said first grid, aircraft elevator control means responsive to the anode currents of said electron discharge devices, means for varying the biasing potential on said second grid to determine a desired climb or glide for the aircraft, a voltage dividing means, a channel for impressing the voltage produced by said voltage dividing means upon said first grid, and means movable as a function of the movement of the aircraft elevator for controlling said voltage dividing means, the construction being such that a potential is applied to said first grid in a direction to bring its bias to the bias which exists upon said second grid.

9. A control system as in claim 8 in which said voltage impressing channel includes a capacitor.

10. A control system for governing the rate of change of altitude of an aircraft including in combination a bridge network, means for impressing a unidirectional potential across a pair of terminals of said bridge, a pair of thermionic tubes each having a grid, means for connecting the other pair of terminals of the bridge with the respective grids of said tubes, one of said grids being connected to said bridge to provide a fixed grid bias therefor, means for unbalancing said bridge to produce a desired predetermined control signal, means for generating a second signal which is a function of the rate of change of altitude of the aircraft, and means for impressing said second signal upon one of said grids.

11. A control system as in claim 10 including means for generating a third signal which is a function of the rate of adjustment of the elevator control surfaces of an aircraft, and means for impressing said third signal upon one of said grids.

12. A control system for controlling the rate of motion of a moving part including in combination a bridge network, means for impressing a unidirectional potential across a pair of terminals of said bridge, a pair of thermionic tubes each having a grid, means for connecting the other pair of terminals of said bridge to respective grids, means for impressing a predetermined fixed grid bias upon one of said grids, means for unbalancing the bridge to produce a desired predetermined control signal, means for generating a second signal which is a function of the rate of movement of said moving part, and means for impressing said second signal upon one of said grids.

13. A control system as in claim 12 including in combination a control element for controlling the motion of said moving part, means for generating a third signal which is a function of the rate of adjustment of said control element, and means for impressing said third signal upon one of said grids.

14. A control system for controlling a moving part including in combination means for developing a potential proportional to the position of the moving part with respect to a fixed reference. a pair of electron discharge devices each having an anode and a grid, cathodes for each of said electron discharge devices, a self-biasing common resistor for said cathodes, means for impressing a fixed biasing potential upon one of the respective grids, means for impressing a biasing potential'upon the other of said respective grids, means for impressing a potential which is a function of the change in position of said moving part with respect to said fixed reference with respect to time upon said first grid, control means responsive to the anode currents of said electron discharge devices for controlling the movement of said moving part, and means for varying the biasing potential of said second grid to determine a desired rate of change in position of said moving part from said fixed reference.

ROBERT HOFSTADTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,020,275 Beers Nov. 5, 1935 2,025,218 Reinkin Dec. 24, 1935 2,204,290 Alkan June 11, 1940 2,276,816 Bagno Mar. 17, 19-12 2,350,024 Francis May 30, 1044 2,415,429 Kellogg et al Feb. 11. 1947 2.450.907 Newton et a1 Oct. 12, 1948 

